Various devices for internal fixation of bone segments in the human or animal body are known in the art. For instance, pedicle screw and/or hook systems are sometimes used as an adjunct to spinal fusion surgery. Such systems may have a rod-receiving portion and an integral anchor portion, or may be provided with a separate anchor member, especially one that may be pivoted with respect to a rod-receiving member. Although pedicle screw systems, comprising a pedicle screw and a rod-receiving device, are commonly used, it is also possible to anchor a rod-receiving device to the spine with a different type of anchor member, such as a laminar hook. The pedicle screw portion of such a system includes an externally threaded stem and a head portion. The rod-receiving device (also referred to as a coupling device) couples the pedicle screw to a spinal rod by receiving and fixing the head portion of the pedicle screw and the elongate spinal rod (commonly referred to as a distraction rod). Two such systems are inserted into respective vertebrae and adjusted along the spinal rod to distract, de-rotate, and/or stabilize a spinal column, for instance to correct scoliosis or stabilize the spinal column in conjunction with an operation to correct a herniated disk. The pedicle screw does not, by itself, fix the spinal segment, but instead operates as an anchor point to fix the coupling device relative to the spinal segment into which the pedicle screw is driven, with the coupling device in turn receiving the rod extending therethrough. One goal of such a system is to substantially reduce and/or prevent relative motion between the spinal segments that are being fused.
Known pedicle screw systems have several drawbacks. For instance, some pedicle screw systems include rather large and bulky assemblies to secure a rod, thus increasing opportunities for tissue damage in and around the surgical site during installation. Many devices are also difficult to assemble and/or install. These problems become even more significant when implanting a coupling member and anchor member into smaller portions of the spine, such as the cervical vertebrae. For instance, installation of coupling members into adjacent cervical vertebrae often requires the central axis of the anchor member and coupling member of polyaxial pedicle screw systems to be pivoted to more extreme angles relative to each other than required in other vertebral regions, due to the size, proximity, and curvature of the vertebrae in the cervical spine.